refactor(tvix): build Rust projects using crate2nix

Introduces granular dependency builds using crate2nix, bootstrapped
off the generated configuration from the newly introduced
workspace (see cl/7533).

This commit checks in the generated Cargo.nix file which can be
regenerated with a parameterless invocation of `crate2nix generate` in
`//tvix`. I tried generating this in IFD, but it turned out to be
harder than what seemed worthwhile for now.

In this setup, the various build targets for Rust projects end up
being attributes of the imported `Cargo.nix` file at the `tvix.crates`
attribute. These still lack configuration, however, which has been
fixed in the various `default.nix` files of individual projects.

Note that we (temporarily) lose the ability to build tvix-eval's
benchmarks in CI. I haven't figured out what magic incantation summons
them from the void again ...

The `eval-okay-readDir` tests from both test suites have been disabled
because they fail for unknown reasons when run in this new derivation.
Somebody will have to debug it!

Change-Id: I2014614ccb9c8951aedbd71df7966ca191a13695
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7538
Autosubmit: tazjin <tazjin@tvl.su>
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

corp/tvixbolt/default.nix

@@ -46,7 +46,7 @@ let
 
      [dependencies.tvix-eval]
     -path = "../../tvix/eval"
-    +path = "${depot.tvix.eval.src}"
+    +path = "${depot.tvix.crates.workspaceMembers.tvix-eval.build.src}"
      default-features = false
 
      [dependencies.serde]

tvix/README.md

@@ -6,6 +6,23 @@ are interested in people who would like to help us review designs,
 brainstorm and describe requirements that we may not yet have
 considered.
 
+## Rust projects
+
+Some parts of Tvix are written in Rust. To simplify the dependency
+management on the Nix side of these builds, we use `crate2nix` in a
+single Rust workspace in `//tvix` to maintain the Nix build
+configuration.
+
+When making changes to Cargo dependency configuration in any of the
+Rust projects under `//tvix`, be sure to run `crate2nix generate` in
+`//tvix` itself and commit the changes to the generated `Cargo.nix`
+file.
+
+`crate2nix` is available via `direnv` inside of depot, or can be built
+from the `third_party.nixpkgs.crate2nix` attribute of depot. Make sure
+to build it from depot to avoid generating files with a different
+version that might have different output.
+
 ## License structure
 
 All code implemented for Tvix is licensed under the GPL-3.0, with the

tvix/default.nix

@@ -1,14 +1,10 @@
 # Nix helpers for projects under //tvix
-
 { pkgs, ... }:
 
 {
-  # Construct a sparse tree for naersk's `root` field, for
-  # compatibility with the workspace-workaround (see top-level comment
-  # in //tvix/Cargo.toml)
-  naerskRootFor = cargoToml: pkgs.runCommand "sparse-tvix-root" { } ''
-    mkdir $out
-    cp -aT ${./Cargo.lock} $out/Cargo.lock
-    cp -aT ${cargoToml} $out/Cargo.toml
-  '';
+  # Load the crate2nix crate tree.
+  crates = import ./Cargo.nix {
+    inherit pkgs;
+    nixpkgs = pkgs.path;
+  };
 }

tvix/eval/default.nix

@@ -1,34 +1,9 @@
-{ depot, pkgs, lib, ... }:
+# TODO: find a way to build the benchmarks via crate2nix
+{ depot, pkgs, ... }:
 
-lib.fix (self: depot.third_party.naersk.buildPackage (lib.fix (naerskArgs: {
-  src = depot.third_party.gitignoreSource ./.;
-  # see https://github.com/nix-community/naersk/issues/169
-  root = depot.tvix.naerskRootFor ./Cargo.toml;
+depot.tvix.crates.workspaceMembers.tvix-eval.build.override {
+  runTests = true;
 
-  doCheck = true;
-
-  # Tell the test suite where to find upstream nix, to compare eval results
-  # against
-  NIX_INSTANTIATE_BINARY_PATH = "${pkgs.nix}/bin/nix-instantiate";
-
-  meta.ci.targets = builtins.attrNames self.passthru;
-
-  copySources = [
-    "builtin-macros"
-  ];
-
-  passthru.benchmarks = depot.third_party.naersk.buildPackage (naerskArgs // {
-    name = "tvix-eval-benchmarks";
-
-    doCheck = false;
-
-    cargoBuildOptions = opts: opts ++ [ "--benches" ];
-
-    copyBinsFilter = ''
-      select(.reason == "compiler-artifact" and any(.target.kind[] == "bench"; .))
-    '';
-
-    passthru = { };
-  });
-}))
-)
+  # Make C++ Nix available, to compare eval results against.
+  testInputs = [ pkgs.nix ];
+}

tvix/nix_cli/default.nix

@@ -1,8 +1,5 @@
-{ depot, pkgs, ... }:
+{ depot, ... }:
 
-depot.third_party.naersk.buildPackage {
-  src = ./.;
-  root = depot.tvix.naerskRootFor ./Cargo.toml;
-  doDoc = false;
-  doCheck = true;
+depot.tvix.crates.workspaceMembers.nix-cli.build.override {
+  runTests = true;
 }

tvix/store/default.nix

@@ -1,19 +1,27 @@
-{ depot, pkgs, lib, ... }:
+{ depot, pkgs, ... }:
 
 let
   protoRoot = depot.nix.sparseTree depot.path.origSrc [
     ./protos/castore.proto
     ./protos/pathinfo.proto
   ];
+
+  protobufDep = prev: (prev.nativeBuildInputs or [ ]) ++ [ pkgs.protobuf ];
 in
-depot.third_party.naersk.buildPackage {
-  src = depot.third_party.gitignoreSource ./.;
-  # see https://github.com/nix-community/naersk/issues/169
-  root = depot.tvix.naerskRootFor ./Cargo.toml;
+depot.tvix.crates.workspaceMembers.tvix-store.build.override {
+  # Ensure protobuf dependencies are available.
+  # TODO: figure out a way to embed this directly in the //tvix
+  # crate2nix config.
+  crateOverrides = {
+    prost-build = prev: {
+      nativeBuildInputs = protobufDep prev;
+    };
 
-  nativeBuildInputs = [ pkgs.protobuf ];
+    tvix-store = prev: {
+      PROTO_ROOT = protoRoot;
+      nativeBuildInputs = protobufDep prev;
+    };
+  };
 
-  PROTO_ROOT = protoRoot;
-
-  doCheck = true;
+  runTests = true;
 }